1. Field of the Invention
The present invention relates to an apparatus for biological treatment of water in which a water containing organic matter (e.g. waste water containing organic matter, recovered water containing organic matter or untreated water in water treatment works) is subjected to biological treatment using aerobic microorganisms to decompose and remove the organic matter, as well as to a method for biological treatment of water, using said apparatus. More particularly, the present invention relates to an apparatus for biological treatment of water which can be suitably incorporated into, for example, a recovery system for the ultrapure water used in semiconductor industry, etc., which gives a short rise time from operational start to stable operation, which can conduct biological treatment stably over a long period of time, and which is easy to maintain, as well as to a method for biological treatment of water using said apparatus.
2. Description of the Prior Art
For the biological treatment of an organic matter-containing water to remove the organic matter, active researches are under way on a biological activated carbon method which uses activated carbon as a carrier for microorganisms. This method is said to allow for the treatment of difficult-to-decompose substances and the biological treatment of higher efficiency because, with the method, the time of contact between microorganisms and substrate is longer and inhibitory factors for proliferation of the microorganisms are removed by adsorption [see, for example, WATER RESEARCH, Vol. 26, No. 10, p. 1389-1401 (1992)].
In the biological activated carbon method, there has been used, as the carrier, spherical or granular activated carbon, and the operation for biological treatment of water has been conducted by a fluidized bed method (up flow) or a fixed bed method (down flow). Since the fixed bed method (down flow) has an advantage of very easy maintenance of apparatus but has a serious problem in that the microorganism film formed on the carrier (activated carbon) gets thicker in a relatively short time and causes clogging, the fluidized bed method (up flow) is in general use. The fluidized bed method, however, has the following drawbacks.
(1) Aeration washing may cause the partial leaving of activated carbon (carrier) from the treatment vessel, making difficult the cleaning of the treating vessel. Meanwhile, operation over a long period of time without washing results in the proliferation of too large a number of microorganisms in the treatment vessel, which adversely affects the capability (treatability) of the apparatus.
(2) Pulverization of activated carbon takes place. It invites clogging of membrane separator when such a membrane separator is installed downstream of the treatment vessel.
(3) Formation of microorganism film on activated carbon allows the activated carbon to agglomerate or become lumpy, resulting in adhesion of activated carbon onto the vessel wall, etc. As a result, the flow of the water to be treated tends to become nonuniform (channeling) in the treatment vessel, and the treatment capability of the apparatus is reduced.
(4) It is difficult to keep activated carbon in a desired fluid state, requiring a complicated maintenance procedure for stable operation.
(5) In order to keep a desired fluid state, it is necessary to supply a large amount of a water to be treated to the treatment vessel. Consequently, in many cases, a circulation vessel to conduct aeration of the water (indirect aeration) is separately provided, in addition to the treatment vessel, which leads to a larger and more complicated apparatus.
(6) In the fluidized bed, since the particles of active carbon of fluidized state as carrier collide with each other, the microorganism film formed on each particle tends to peel off from the particle surface, as compared with the case of the fixed bed. Consequently, the time from operational start to stable operation, i.e., the rise time, is longer particularly when the proliferation rate of microorganisms is small as in the case of treating water of low organic matter content, for example, water recovered from the washing operation using ultrapure water in the semiconductor industry.
In contrast, the fixed bed method has no drawbacks inherent in the fluidized bed method as mentioned above. However, the fixed bed has a serious problem in that the microorganism film formed on activated carbon gets thicker in a relatively short time and causes clogging of the packing medium composed of the activated carbon. Thus, the conventional fixed bed method allows for no efficient biological treatment.